1. Field of the Invention
The present invention relates to a mode-locked semiconductor laser that generates ultra-short light pulses that can be used for, for example, optical data processing or optical communication.
2. Description of the Related Art
Mode-locked semiconductor lasers are expected to serve as a means of easily obtaining ultra-fast light pulses which are useful for such applications as optical communication or optical information processing systems. The two forms of realizing mode-locked semiconductor lasers are passively mode-locked semiconductor lasers and actively mode-locked semiconductor lasers.
As shown in FIG. 1, this example includes a waveguide structure which is electrically divided into a gain region 21 and saturable absorption region 22 with current being injected or a bias voltage being applied independently to each region.
Saturable absorption region 22 possesses a saturation characteristic whereby its own absorption coefficient decreases by absorption of light when an appropriate reverse bias voltage is applied.
The end surface of gain region 21 is a facet produced by, for example, cleavage, that contacts the air and functions as a reflecting mirror having a reflectivity of approximately 30%. In contrast, the end surface on the side of saturable absorption region 22 is a reflecting mirror 26 having a reflectivity approaching 100% that is produced by coating a dielectric multilayer film, and forms an optical resonator structure.
It is known that, a semiconductor laser of this configuration makes a self-pulsating operation on a self-starting mode-locking operation by adjusting the current injected into gain region 21 and the reverse bias voltage applied to saturable absorption region 22.
As shown in FIG. 2, this example of the prior art includes a waveguide structure 38 made up of three regions: gain region 31, saturable absorption region 32, and gain modulation region 33. According to this configuration, gain region 31, saturable absorption region 32, and gain modulation region 33 are electrically isolated each other, and current is injected or a bias voltage is applied separately to each region.
Moreover, both end surfaces of waveguide structure 38 are end facets surfaces produced by, for example, cleavage, that contact the air and function as reflecting mirrors having a reflectivity of approximately 30%, thereby forming an optical resonator structure.
Current is injected from a direct current source to gain region 31. Reverse bias voltage is applied to saturable absorption region 32. Forward bias voltage modulated at a period that is an integer power of the round-trip time of light in the optical resonator is applied to gain modulation region 33 from an external power source.
It is known that in a semiconductor laser of this configuration, mode-locking operation can be achieved by adjusting the current injected to gain region 31, the reverse bias voltage applied to saturable absorption region 32, as well as the modulation depth and frequency of the current injected to gain modulation region 33.
The two examples of the prior art described hereinabove are monolithic mode-locked semiconductor lasers, where on optical resonator is made on the waveguide structure by two end cleaved facets. However, another form also exists in which one or two of the above-described cleaved facets are replaced by a diffraction grating that functions as an external wavelength-selective reflecting mirror, thereby forming an external resonator. In an external resonator, the round-trip time within the resonator can be extended by lengthening the resonator (cavity), thereby allowing a low repetition rate of pulsed light to be obtained.
Normally, the typical repetition rate is more than 10 GHz in a monolithic mode-locked semiconductor laser.
However, the above-described mode-locked semiconductor lasers of the prior art have the following drawbacks:
(1) For passively mode-locked semiconductor lasers, the structure of the laser must be adequately optimized to meet the self-starting conditions of the mode-locking operation. However, since the design principles for such optimization are not yet clearly understood, the manufacture of a sample having a variety of structures entails a process of trial and error in order to arrive at an element that can satisfy self-starting conditions, and as a result, the yield rate for elements is poor and design modification of elements has been difficult.
(2) For passively mode-locked semiconductor lasers, even if an element that can meet the self-starting conditions is obtained, the mode-locking conditions are constrained, and use under desired conditions (power, repetition, etc.) has therefore been difficult.
(3) For actively mode-locked semiconductor lasers, the mode-locking conditions are again constrained, and use under desired conditions (power, repetition, etc.) has been difficult.
(4) Both passively and actively mode-locked semiconductor lasers have low stability of pulse repetition rate and a high level of jitter.
These problems must be solved if mode-locked semiconductor lasers are to serve as a clock source in optical information processing systems.